Blood Alcohol (BAC) Calculator

This calculator uses the Widmark formula, the method most BAC calculators and many police and court experts use for a rough estimate. It works out how much pure alcohol you drank, spreads it through the water in your body, then takes off what your liver has cleared since your first drink. It assumes all the alcohol has been absorbed. In real life it takes about 30 to 90 minutes after a drink to reach your blood (longer after food), so your level can still be rising soon after drinking.

Not medical or legal advice. Real blood alcohol varies a lot with your body, food, health, medicines, tiredness and how fast you drank. This tool must never be used to decide whether you can drive, work, swim or look after children. Only a breath or blood test measures BAC, and even below a legal limit you can be impaired. The only safe level for driving is zero.

The formula

Grams of alcohol = size in ml × ABV ÷ 100 × 0.789, added up for every drink
Peak BAC (%) = grams ÷ (r × weight in kg × 1,000) × 100
BAC now = peak − 0.015 × hours since first drink (never below 0)
Hours to about 0.00 = BAC now ÷ 0.015
Hours to below a limit = (BAC now − limit) ÷ 0.015
Watson r = body water ÷ (0.8 × weight), where body water (litres) =
  men: 2.447 − 0.09516 × age + 0.1074 × height cm + 0.3362 × weight kg
  women: −2.097 + 0.1069 × height cm + 0.2466 × weight kg

0.789 g per ml is the density of alcohol. The r factor is the share of your body that holds alcohol: about 0.68 for men and 0.55 for women on average, because women tend to have less body water for the same weight. 0.8 is the share of blood that is water. 1% BAC means 1 g of alcohol in 100 ml of blood, so 0.08% = 0.8 g/L = 80 mg/100 ml (0.8‰).

Worked example

A man who weighs 70 kg has 2 beers of 330 ml at 5% and a 150 ml glass of wine at 12%, and his first drink was 2 hours ago. Pure alcohol: 2 × 330 × 0.05 = 33 ml plus 150 × 0.12 = 18 ml, so 51 ml × 0.789 = 40.239 g. Peak BAC: 40.239 ÷ (0.68 × 70 × 1,000) × 100 = 40.239 ÷ 47,600 × 100 = 0.0845%. In 2 hours his liver clears about 0.015 × 2 = 0.030%, so his estimated BAC now is 0.0845 − 0.030 = 0.0545% (about 0.55 g/L or 55 mg/100 ml). Reaching about 0.00 takes 0.0545 ÷ 0.015 = 3.64 hours (about 3 h 38 min) more. Getting under a 0.05% limit takes (0.0545 − 0.05) ÷ 0.015 = 0.30 hours, about 18 minutes, but he could still be impaired, and the only safe level for driving is zero. That is 40.239 ÷ 14 = 2.9 US standard drinks, 51 ÷ 10 = 5.1 UK units, or 4.0 Australian standard drinks, and 40.239 × 7 = about 282 kcal from the alcohol alone.

Standard drinks around the world

Drink-drive limits by country

Blood alcohol limits as % (grams per 100 ml of blood). Many countries test breath instead and use a matching breath limit. Laws change and some places set lower limits for young, new, commercial or professional drivers, so always check the official rules where you are. The limits table is waiting for its next check against official sources; figures not yet confirmed are shown as given by the WHO global road safety report. Even under any of these limits, alcohol slows your reactions: the only safe amount to drink before driving is none.

What different levels can do

A rough general guide based on NIAAA guidance. People who drink often may feel less drunk, but their reactions and judgement are still affected. Mixing alcohol with medicines or other drugs makes it more dangerous.

Why your real level may be different

• Food: eating slows how fast alcohol reaches your blood, so the peak is lower and later, but it doesn't stop it.
• Body make-up: fat holds little water, so two people of the same weight can have different r factors. The Watson method in Advanced uses age and height to allow for this a little.
• Speed of drinking: drinks taken quickly push your level higher than the same drinks spread out.
• Liver and health: clearance varies from about 0.010 to 0.020% an hour and can be slower with liver problems or some medicines.
• Pour size: home and bar measures are often larger than "standard", and strong craft beers and wines can be 8–15%.

Worried about your drinking? Talk to your doctor, or look up a free local support service. In the UK the NHS says not to regularly drink more than 14 units a week, spread over 3 or more days.

Sources: NIAAA standard drink, NHS alcohol units, NHS alcohol poisoning, GOV.UK drink-drive limit, WHO Global status report on road safety 2023. Watson, Watson and Batt (1980), American Journal of Clinical Nutrition 33:27–39. Figures waiting for their next check.